Neonates have a limited capacity for fat digestion. Human milk fat contains significant quantities of triglycerides with medium chain fatty acids, which are readily digestible, and provides a major source of calories and essential fatty acid for the neonate. The amount of triglyceride containing medium chain fatty acid increases 3-fold in human milk during early lactation; however little is known as to the mechanism regulating, or the significance of this change. We have shown with rats that biosynthesis of medium chain fatty acid in vitro, requires two enzymes, fatty acid synthetase and thioesterase II. We propose to determine whether the level of fatty acid synthetase and thioesterase II in rat mammary tissue correlates with the capacity of the gland, at various stages of development, for medium chain fatty acid biosynthesis. Tissue from virgin, pregnant, lactating and weaned rats will be examined for lipogenic capacity and type of fatty acids synthesized. The presence of fatty acid synthetase and thioesterase II will be determined enzymatically, immunologically and histologically. Regulation of the turnover of the enzymes will be studied using an isotopic immunochemical procedure. We propose to determine, in humans, whether the change in milk triglyceride fatty acid composition during lactation is due to a change in the contribution of mammary gland lipogenesis to milk fat production, or a change in the type of fatty acids synthesized, de novo, by the gland. Milk samples will be collected from women at various stages of lactation and the triglyceride composition determined. Epithelial cells will be purified from milk and their capacity for lipogenesis determined in isotopic incorporation experiments; the type of fatty acids synthesized will be determined and the activities of fatty acid synthetase and thioesterase II assayed. Data will be analyzed according to several criteria, including maternal age and dietary habits, and length of gestation period.